An apparatus for the control of motor vehicle tire pressure can comprise a compressor which can be connected to the tires by respective valves and provided with an electronic controller which, via sensors, detects the tire pressures and the operating state of the vehicle and regulates pneumatic valves between the compressor and the tire for relating the tire pressures based upon such conditions as the vehicle speed, the vehicle load, the road conditions, the terrain and the like.
Austrian patent 408,867, for example, discloses such a device for the automatic actuation of a tire-filling system for a motor vehicle in which a compressor is connected to the tires and the pressure reservoir.
The principal parameter which controls the tire pressure is the vehicle speed and the actual tire pressure can be measured and a setpoint for the tire pressure can be calculated from the vehicle speed and the deviation of the actual tire pressure from the setpoint can be corrected by admitting fluid to or reducing fluid from the tire via a respective valve until the setpoint of the tire pressure is reached for each tire provided with such control.
The air required for filling the tires or raising the tire pressure is usually drawn by the compressor from the ambient atmosphere and compressed and the release of air for reduction of the tire pressure is also to the surrounding atmosphere.
The critical operation of an automatic tire-pressure controller is the filling of the tires with air to the proper pressure, since, at any vehicle speed, operation of the tire below the optimum pressure creates a safety problem. For systems utilizing a storage reservoir for the fluid under pressure, automatic operation in this manner can frequently result in rapid depletion because of multiple control operations and thus place the system in a state where it is unable to reliably control the pressure of the tires as a function of the vehicle speed, for example.
To avoid such critical conditions, the compressor can be connected in a system between the tires and the pressure reservoir which is normally closed, i.e. neither draws fluid from nor discharges fluid into the atmosphere. In that case, the pressure reservoir may be a reserve wheel and the compressor is connected to that pressure reservoir by a 3-port 2-position (3/2) magnetic valve. This magnetic valve however must be controlled generally independently of the remainder of the system and the valve tends to increase the gas of the system and the need for operating this valve increases its complexity.